1. Field of the Invention
This invention is directed to a circuit controller for protecting a load and a power circuit connected to the load, and more particularly to such an integrally packaged electrical circuit controller connected to a motor and having a contactor, a circuit breaker and a current throttle for providing current limiting protection.
2. Description of the Prior Art
A. CONTACTORS
Electromagnetic contactors are well-known in the art. A typical example may be found in U.S. Pat. No. 3,339,161 issued Aug. 29, 1967 to J. P. Conner et al. entitled "Electromagnetic Contactor" and assigned to the assignee of the present invention. Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching and similar applications. A motor-starting contactor with an overload relay system is called a motor controller or starter.
A contactor usually has a magnetic circuit which includes a fixed magnet and a movable magnet or armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage which may be interconnected with the main contacts of the contactor for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap. Disposed on the armature is a set of bridging contacts, the complements of which are fixedly disposed within the contactor case for being engaged thereby as the magnetic circuit is energized and the armature is moved. The load and voltage source therefor are usually interconnected with the fixed contacts and become interconnected with each other as the bridging contacts make with the fixed contacts.
Generally, as the armature is accelerated towards the magnet, it must overcome two spring forces. The first spring force is provided by a kickout spring which is subsequently utilized to disengage the contacts by moving the armature in the opposite direction when the power applied to the coil has been removed. As this occurs, the contacts are opened. The other spring force is provided by a contact spring which begins to compress as the bridging contacts abut the fixed contacts. The force of the contact spring determines the amount of electrical current which can be carried by the closed contacts, and furthermore determines how much contact wear is tolerable as repeated operation of the contactor occurs. It is usually desirous for the contact spring to be as forceful as possible, thus increasing the current-carrying capability of the contactor and increasing the capability to adapt for contact wear. However, since this force must be overcome by the energy provided to the electromagnet during the closing operation, more closing energy will generally be required for relatively stiffer contact springs than for less stiff contact springs.
The addition of an overload relay transforms a contactor into a starter or motor controller. The purpose of a thermal overload relay is to generate and sense heat produced by line current and "trip" (stop the motor) if the retained heat exceeds an acceptable level. The function of a conventional thermal overload relay in a starter is to generate heat in a heater using the current flowing to the motor and the resistance of the heater element. This heat is directed toward either a bimetal or eutectic alloy that "trips" and opens the starter under overload conditions.
The more heat generated in a starter, the greater the physical size required to dissipate the heat. Also, the more space must be left around the starter to avoid injurious effects to surrounding devices.
In traditional starters, heat is generated from three sources: (1) coil operation; (2) current through the contacts; and (3) overload relay heaters. Traditional starters have thus been improved in three ways: (1) low coil holding power reduces heating in the coil; (2) high contact force results in less heat generated in the contact set; and (3) current sensors, rather than heaters, eliminate most of the temperature rise in the overload relay.
In conventional starters, the limiting factor in establishing short circuit withstand ratings is primarily the heaters. Heaters have a maximum amount of current that they can withstand without melting open or losing calibration.
In contrast to heaters, current sensors output a voltage proportional to the change in current. After an analog-to-digital conversion of the voltage, a microprocessor squares and integrates the converted digital value to achieve a true measure of motor heating. This approach allows for a linear motor protection curve and provides an accurate degree of protection.
A typical example of a starter utilizing a current sensor may be found in U.S. Pat. No. 4,893,102 issued Jan. 9, 1990 to James A. Bauer entitled "Electromagnetic Contactor with Energy Balanced Closing System" and assigned to the assignee of the present invention, which is herein incorporated by reference.
Current sensors and related circuitry are immune to damage by high currents and so are not the limiting factors in establishing short circuit withstand ratings. The current sensor simply saturates under high current conditions, limits the voltage signal transmitted to the analog-to-digital converter and, in turn, the microprocessor. Whenever overload protection is built in, the starter may be the same size as the contactor and, therefore, be much smaller than conventional starters. Smaller physical size combined with reduced heat offers the possibility of: (1) reducing enclosure size and associated cost; (2) more densely populating enclosures, further reducing enclosure cost; and (3) retrofitting existing motor control.
A Class II ground-fault protective starter may sense and respond to low-level and arcing ground-faults often occurring in motor branch circuits. Such a starter opens the circuit with the ground fault, provided the magnitude of the fault current is within the interrupting capability of the device. A branch circuit short circuit protective device clears faults that exceed the interrupting rating of the starter. Such additional catastrophic short circuit withstand protection is generally provided by a separate circuit breaker that is connected in series with the phases of the power line and the contactor. Thus, the circuit breaker is generally the limiting factor in determining the worst case short circuit current that would damage or degrade the contactor and other components of the power circuit.
B. CIRCUIT BREAKERS
Molded case circuit breakers are generally old and well- known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit condition. An overload condition is normally about 125-600 percent of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000 percent or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include at least one pair of separable contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. A moving contact assembly provides continuity between line and load terminals when the circuit breaker is on. When the circuit breaker trips or is switched off, the moving contact assembly moves away from a stationary contact or contacts.
Trip mechanisms generally provide automatic (thermal and magnetic) and manual (pushbutton) modes to trip the circuit breaker. The thermal and magnetic elements of circuit breakers can be adjusted, for example, by rotating adjustment buttons in the cover of the circuit breaker to a desired setting.
The thermal trip mechanism operates in response to overload conditions. A bimetal element is part of the current carrying path. When there is an overload, the increased current flow heats the bimetal and causes it to bend. As the bimetal bends, it touches and rotates a trip bar causing the circuit breaker to trip. The time needed for the bimetal to bend and trip the circuit breaker varies inversely with the current.
The magnetic trip mechanism operates when there is a high current (short circuit) in the current path. The mechanism includes an electromagnet and an armature. When high level current passes through the conductor, the magnetic field strength of the electromagnet rapidly increases and attracts the armature. As the top of the armature is drawn to the electromagnet, the armature rotates the trip bar causing the circuit breaker to trip.
The pushbutton mechanism provides a manual mode of tripping the circuit breaker by depressing a button located in the circuit breaker cover. When the pushbutton is pressed, a plunger rotates the trip bar causing the circuit breaker to trip.
In the automatic mode of operation, the contacts may be opened by an operating mechanism, controlled by an electronic trip unit, or by magnetic repulsion forces generated between the stationary and movable contacts during relatively high levels of overcurrent.
In one automatic mode of operation, the contact assemblies for all poles are tripped together by an electronic trip unit and a mechanical operating mechanism. More particularly, the electronic trip unit is provided with current sensors to sense an overcurrent condition. When an overcurrent condition is sensed, the current transformers provide a signal to the electronic circuitry within the electronic trip unit to actuate the operating mechanism to cause the main contacts to be separated.
In another automatic mode of operation, the contact arm assemblies are disengaged from the mechanical operating mechanism and are blown open by magnetic repulsion forces. More particularly, magnetic repulsion members or shunts are used to allow the contact arm, which carries the movable main contact, to pivot. Each magnetic repulsion member is generally U-shaped defining two legs. During relatively high level overcurrent conditions, magnetic repulsion forces are generated between the legs of the magnetic repulsion member as a result of current flowing through the legs in opposite directions. At a relatively high level overcurrent condition, these magnetic repulsion forces cause the contact arm carrying the movable main contact to be blown open.
During a blow open condition, each contact arm is operated independently of the mechanical operating mechanism. For example, for a three phase circuit breaker having a high level overcurrent on the A phase, only the A phase contact arm will be blown open by its respective repulsion member. The contact arms for the B and C phases would remain closed and, thus, would be unaffected by the operation of the A phase. The contact arms for the B and C phases are, however, tripped by the electronic trip unit and the operating mechanism. This is done to prevent a condition known as single phasing, which can occur for circuit breakers connected to rotational loads, such as motors. In such a situation, unless all phases are tripped, the motor may act as a generator and contribute to the overcurrent condition. An example of a circuit breaker providing blow open operation may be found in copending U.S. patent application Ser. No. 07/779,441 filed Oct. 13, 1991 by Ronald W. Crookston et al. entitled "Molded Case Current Limiting Circuit Breaker" and assigned to the assignee of the present invention.
A circuit breaker also includes a cradle having latch and reset surfaces for latching and resetting the operating mechanism. A molded case circuit breaker further includes a molded base and a coextensive cover. A centrally located aperture is provided in the cover for receiving an operating handle to allow the circuit breaker to be operated manually. The handle is comprised of an arcuate shaped base portion with a radially extending handle portion.
A common type of circuit breaker has a handle which moves linearly between an on and an off position. The handle is connected to the movable contacts of the circuit breaker through a spring powered, over center toggle device which trips the contacts open and moves the handle to an intermediate position in response to certain overcurrent conditions. This type of circuit breaker may be found in U.S. Pat. No. 4,725,800 to Kurt A. Grunert et al. entitled "Circuit Breaker with Magnetic Shunt Hold Back Circuit" and assigned to the assignee of the present invention, which is herein incorporated by reference.
Another type of circuit breaker has a rotary handle which may be found in U.S. Pat. No. 5,219,070 to Kurt A. Grunert et al. entitled "Lockable Rotary Handle Operator for Circuit Breaker" and assigned to the assignee of the present invention, which is herein incorporated by reference.
In some installations, circuit breakers are mounted behind a panel or behind a door in a cabinet. Typically in these installations, the handles of the circuit breakers protrude through openings in the panel or door and are operated directly. In other installations, the rotary handle is remotely located from the circuit breaker by a shaft connecting the rotary handle to the circuit breaker.
C. POWER CIRCUIT FAULT CURRENTS
In conventional installations, a circuit breaker is connected to one or more contactors or starters to clear faults in the power circuit wiring between the circuit breaker, the starters and the motor. However, the use of multiple starters with an individual circuit breaker increases the normal current carrying capacity requirement of the circuit breaker. Furthermore, the magnitude of the potential fault current and the probability of circuit faults are increased with the increased current carrying capacity required by the circuit breaker. Hence, the increased capacity, increased wiring requirements and larger physical layout of the power circuit increase the likelihood of a catastrophic circuit fault. Moreover, improvements in modern power distribution systems have enabled power sources to supply greater magnitudes of potential fault current.
In prior art systems, under fault conditions involving a line-to-line or line-to-ground fault, excessive current may flow from the alternating current power source. Such excessive current could flow through the circuit breaker, the power circuit wiring, the contactor and the motor. Because of the trip characteristics of the contactor, which is merely designed for interrupting currents associated with a motor load or overload, the circuit breaker acts first to protect the circuit. However, high level short circuit faults may, nevertheless, cause damage, reduced component life or excessive visual display.
There remains a need therefore for an improved circuit controller that will reduce the potential for fault current damage in the case of a severe electrical disruption.
There is a more particular need for an improved circuit controller that combines a circuit breaker, a current throttle and a contactor having an overload relay, the combination acting in unison during a potentially massive fault current, so that each device complements the other devices and also has the capability of being reentered into usable service, with no rehabilitation.
There is a further need for an improved circuit controller that will limit the potential fault current from a modern power distribution system without increasing the complexity and the cost of the circuit breaker.
There is a more particular need for an improved circuit controller that is housed in an integral, compact modular unit for the coordination of fault damage control without providing additional circuit wiring.
There is still a further need for an improved circuit controller having a current throttle impedance that allows normal rated load current to flow indefinitely with no deleterious effect but, also, substantially limits current flow in case of a high potential fault current such as a 100 KA, three phase bolted fault current without current throttle limitation.
There is more particular need for an improved circuit controller having a current throttle that minimizes resistive power losses and heat generation.
There is also a need for an improved circuit controller that provides a user-configurable current throttle, for limiting fault current, that provides an additional impedance beyond that of the circuit wiring.
There is still a further need for an improved circuit controller having a current throttle that has a built in thermal transfer capability for heat that is generated.
There is yet another need for an improved circuit controller having a current throttle that is housed in a protective, concealed and secure enclosure to maintain a stable coil-form in the enclosure and to enhance user safety and security, for operators and examining personnel, from exposed electrical conductors and heated components.